The present application describes calibration of A/D converters, and specifically calibration of successive approximation A/D converters using the same capacitors that are used for signal conversion.
Many A/D converters depend on comparators to make their conversion. Each A/D converter can have an inherent offset caused by the operation of its comparators. One use of A/D converters is in image sensors. The image sensor produces an analog output which is buffered by a source follower, and then is converted to digital by the A/D converter. The comparator offset often exceeds the required level of sensitivity from the A/D converter. Calibration improves the sensitivity.
The calibration is often carried out by attaching a number of additional capacitors to a signal line, and using those capacitors to calibrate the A/D converter.
This may result in an increasing capacitive load for a column source follower in the image sensor. Moreover, this takes up some percentage of the precious real estate on the chip. This is often a difficult issue in these A/D converters. This kind of system may take up some of the area. The separate circuitry for control of these capacitors may also require additional control signals. This may also itself take up precious area.
The present application describes using the same capacitors for calibration and to carry out the work that is done by the A/D converter. An additional control unit may be used with an additional latch to memorize an offset level.